US3617643A - Time division switching system employing common transmission highways - Google Patents

Time division switching system employing common transmission highways Download PDF

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US3617643A
US3617643A US844945A US3617643DA US3617643A US 3617643 A US3617643 A US 3617643A US 844945 A US844945 A US 844945A US 3617643D A US3617643D A US 3617643DA US 3617643 A US3617643 A US 3617643A
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lines
highways
signal
highway
time slot
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Walter R Nordquist
Wing N Toy
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AT&T Corp
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Bell Telephone Laboratories Inc
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04Q—SELECTING
    • H04Q11/00—Selecting arrangements for multiplex systems
    • H04Q11/04—Selecting arrangements for multiplex systems for time-division multiplexing

Definitions

  • PBX Electronic private branch exchange system
  • PBX the electronic private branch exchange system
  • All of the switch units are served in common by a remote control unit which, because of its electronic components, can tolerate many times the amount of traffic that a single switch unit can provide.
  • the line capacity of a switch unit in such a system is dictated instead by the nature of the internal time division switching operation.
  • the sampling frequency in a time division multiplex system must be at least twice the highest frequency of the signal to be transmitted. For voice, this requires a sampling rate of approximately 8 kilohertz, and typical PBX systems employ 24 time slots recurring at l25-microsecond intervals termed frames, the time slot duration being 5.2 microseconds.
  • the energy transferred during operation of the time divi sion switch must be approximately equal to the energy in the signal between successive samples in order to permit faithful reproduction of the original signal at the receiving terminal.
  • the time slot duration is made smaller, more power must be transmitted through the time division switch, and it is this situation which provides the ultimate limitation on the maximum number of time slots per frame. Such a limitation, of course, severely restricts the size of a switch unit on a customers premises, and expansion beyond that limit may require additional switch units which in turn adds to the expense of the PBX facility.
  • the problem encountered with respect to the capacity of a PBX switch unit on a customer's premises is solved by employment of a unique switching arrangement which permits a reduction in the duration of the time division switching opera tion itself, thus realizing a significant increase in the number of conversations which may time-share the common transmission highway.
  • Signals from a subscriber station are sampied at a rate which is at least twice the highest frequency component. Thus, assuming the highest frequency in a speech signal is 4 kilohertz, sampling will be effected at an 8-kilohertz rate.
  • the speech samples then are coded and applied to a transceiver which may comprise a first register having sufficient capacity to accommodate all of the digits in the coded sample and being capable of bilateral transmission.
  • the output of the transceiver is applied to another register on a common transmission highway via a bidirectional transmission gate which is enabled selectively during a brief portion of a time slot.
  • the sample on the common highway is transferred through another bidirectional gate, in a later portion of the same time slot, to another transceiver connected to the called subscriber station. Subsequent to this time slot, the sample is decoded and applied to the called subscriber station.
  • a similar path ex- LII ists for transmission of signal samples from the called station to the calling station including a second common highway.
  • Such an arrangement permits a reduction in the duration of a time slot to the extent that, with a sampling rate of 8 kilohertz, more than 1,000 time slots may be served within a repetitive l25-microsecond frame. This contrasts with the permissible 24 time slots in prior art systems operating at the same sam pling rate.
  • a similar operation is performed in a two-wire system as contrasted with the four-wire arrangement considered hereinbefore.
  • the distinction rests in the fact that separate paths exist for incoming and outgoing signals in the four-wire system while the two-wire system contemplates that the same path will be shared by incoming and outgoing signals. Occasionally in telephone conversations both parties will be speaking simultaneously. In this event the signal transmitted over the common highway in the two-wire system will be a composite of the signals produced at both calling and called stations.
  • the composite signal available on the two-wire path to each of the calling and called subscribers then is altered by a subtraction operation prior to decoding, such that only the desired signal is applied to each of the calling and called stations.
  • the two-wire operation differs from the four-wire operation in the provision of this subtraction technique in each subscriber line plus the addition of the coded signal samples present on the two common highways.
  • the resultant composite signal is returned to both calling and called lines via the corresponding highways by directional gates and transceivers.
  • the method of addition and subtraction depends upon the technique used in quantizing the input signal for encoding in binary form. For example, if the quantization is logarithmic. the addition and subtraction must be done logarithmically.
  • conference connections may be completed in a similar fashion.
  • the composite signal received from all conferees may be accumulated during several time slots and then returned simultaneously to the conferees.
  • the resultant composite signal is simply the summation of all signals received in the common highways during the previous sequence of time slots.
  • FIG. I is a block diagram of a communication system in accordance with one illustrative embodiment of the invention.
  • FIG. 2 is a time chart illustrating the operations occurring during a single time slot in the system depicted in FIG. 1;
  • FIG. 3 is a block diagram of a communication system in accordance with another illustrative embodiment of the invention.
  • FIG. 4 is a time chart illustrating the operations occurring during a single time slot in the system depicted in FIG. 3;
  • FIGS. 59 are block diagrams of systems of the type depicted in FIG. 3 modified to accommodate conference connections and various other operations.
  • FIG. I the time division telephone system depicted in FIG. I is similar to the system disclosed in R. C. Gebhardt et al. US. Pat. No. 3,225,144, issued Dec. 21, I965, which will be described in general terms hereinafter to provide a basis for the detailed description of the improvements realized in accordance with this invention.
  • a PBX switch unit is illustrated which, in accordance with the Gebhardt et al. disclosure, provides the necessary switching and control facilities to accommodate a number of telephone stations -1 through I00-n.
  • a remote control unit processes all information provided by the illustrated switch unit in order to establish all desired call connections through the switch unit. The information necessary to the establishment of the actual time division switching connections is transferred from the remote control unit to the switch store 150 at the switch unit where it is included in a list of gate control messages which are cyclically scanned and applied individually in a regular sequence to memory register 140.
  • the message is translated into gate control signals by translators 130 and 131 to provide a sequence of gate control signals which enable transmission gates represented by gates 107, 107', 108 and 108.
  • Individual samples of the voice signal provided by the calling and called stations active in this particular conversation are thus transferred between the stations via common transmission highways 115 and 120.
  • each of 24 speech sample periods or time slots has a duration of 5.2 microseconds. Such a duration is required to assure proper transfer of the speech samples through the time division switching network and faithful reproduction of the original signal at the receiving terminal.
  • the number of available time slots may be increased by an order of magnitude, and this operation will be described hereinafter.
  • the facilities required in the transmission path include encoders 101 and 103, which convert the original voice signal to a pulse code modulated (PCM) form.
  • PCM pulse code modulated
  • the number of bits required to encode voice signals into PCM form varies between four and eight depending upon the quality desired.
  • m bits are utilized.
  • control gates 107, 107', 108 and 108' and highways 115 and 120 also each accommodate m bits.
  • decoders 102 and 1041 restore the coded voice signals to analog form prior to receipt by the corresponding stations 100-1 and 100-n.
  • Such encoders and decoders for conversion between analog and PCM may take any one of a number of forms available in the art.
  • Transceivers 105 and 106 comprise simple shift registers having a number of register stages corresponding to the number of digits in each coded speech sample. Again such a register is well known in the art.
  • Transmission gates I07, 107', 108 and 100' are of a form for rapid transfer of digital information between transceivers 105, 106 and the common transmission highways 115, 120.
  • these gates should be capable of transmitting digital information in either direction as will be evident in considering the various arrangements depicted in FIGS. -9.
  • Such a bidirectional gate is known in the art as disclosed, for example, in W. R. Nordquist et al. patent application, Ser. No. 787,135 filed Dec. 26, 1968.
  • Transmission highways 115 and 120 are m bit lines which each receive a signal sample in a first portion of a time slot and apply the signal samples to the respective registers 116 and 121, which again are of conventional form.
  • the prior art switching arrangement as described in the aforementioned Gebhardt et al. patent utilizes a single common transmission highway which receives a signal sample from a calling station during one time slot and transmits that sample to the called station during the same time slot. in this instance, therefore, a pair of m bit transmission gates, each corresponding to one of the calling and called stations, operates simultaneously to effect the desired signal transfer between the two stations.
  • the arrangement in accordance with this embodiment of our invention stores each signal sample transferred to one of the two highways 115 and 120 during a first portion of a time slot and directs these stored samples to the appropriate receiving stations in a later portion of the same time slot.
  • switch store 150 contains the designations of the appropriate gates which must operate during the preassigned time slot to effect a signal transfer from these two stations.
  • translators and 131 After a brief delay and still within the preassigned time slot interval, translators and 131 provide control signals simultaneously on leads 133 and 134, serving to enable the respective gates 108' and 107. This operation results in the transfer of signal samples from registers 116 and 121 to transceivers 106 and 105, respectively. Thereafter the signal samples are converted to analog form in decoders 102 and 104 and applied to the respective stations 100-1 and 100-n.
  • each transceiver may be time shared by a number of stations on a time multiplex basis. This, of course, reduces drastically the number of transceivers and transmission gates required in this arrangement.
  • With unilateral transmission it is evident that two paths through the network are involved in each active conversation, one path for transmitting or talking and another path for receiving or listening.
  • the encoded samples provided by stations 100-1 and 100-n are stored in their respective transceivers 105 and 106.
  • the contents of the transceivers are interchanged by means of the transmission gates and common highways, the latter being shared by all of the stations terminating on the switch unit. As indicated, the contents of the transceivers are applied to the decoders after the sampling period has terminated.
  • the preassigned time slot is divided into four operating intervals of 20, 40, 20 and 40 nanoseconds, respectively, for a total of l20-nanosecond time slot.
  • the sequence of operations occurring during the time slot is noted in FIG. 2.
  • registers 116 and 121 on the common highways are cleared or reset to zero preparatory to the receipt of the next coded signal sample.
  • the signal sample available in transceiver 105 is transferred to register 116 via transmission gate 107 and common highway 115.
  • the signal sample in transceiver 106 is transferred to register 121 via transmission gate 108 and common highway 120.
  • transceivers I05. and 106 are cleared or reset to zero preparatory to the return of the signal samples in the final IO-nanosecond interval of the time slot.
  • the signal sample in register 1 I6 is transferred to transceiver 106 via highway 115 and gate 108' while the sample in register 121 is transferred to transceiver 105 via highway 120 and gate 107'.
  • switch store I50 cycles through all of the control words corresponding to time slots in sequence. If the sampling rate is 8 kilohertz, a scan through switch store I50 will be completed in I25 microseconds. Since each time slot requires only 120 nanoseconds, more than 1,000 time slots can be accommodated within the l25-microsecond frame interval. This then contrasts with the 3.2-microsecond time slot in the aforementioned Gebhardt et al. arrangement which can accommodate a mere 24 time slots in the same frame interval.
  • FIG. 1 The arrangement described in regard to FIG. 1 may be termed a four-wire system with all transmission unilateral.
  • An alternative arrangement which proves advantageous for conference connections, as described hereinafter, may be termed a two-wire system in which signal samples are transmitted in both directions through the same transmission path.
  • FIG. 3 The apparatus and interconnecting facilities are essentially the same as those depicted in FIG. I with the addition of a subtraction circuit in each line, such as subtractors 300 and 301 associated with the respective stations I-I and I00-n.
  • This circuit performs a subtraction operation between the coded outgoing signal sample and the composite of the incoming and outgoing signal samples received through the switching network. The resultant of the subtraction is the desired incoming coded signal sample.
  • adder circuit 305 which, as its name implies, serves to add the coded signal sample from each station engaged in a conversation. The resultant sum is applied to sum buffer 310. All of these circuits are straightforward and easily implemented with arrangements available in the art.
  • the manner of operation of the arrangement depicted in FIG. 3 is indicated in the time chart, FIG. 4.
  • the time slot in this instance is divided into five distinct intervals of, respectively, 20, 40, 20, 20 and 40 nanoseconds for a total time slot interval of I40 nanoseconds.
  • the sequence of operations may be traced through the network of FIG. 3, considering again the example of a conversation in progress between stations 100-1 and 100m.
  • highway registers 116 and 121 and sum buffer 310 are cleared by resetting them to zero.
  • the encoded signals stored in transceivers 105 and 106 representing signal samples from the respective stations 100-1 and 100m, are thereafter gated to the respective highway registers 116 and 121 via bidirectional gates 107 and 108. This action occurs during the second distinct interval in the time slot which has a duration of 40 nanoseconds, FIG. 4.
  • the signal samples currently stored in registers 116 and I21 are applied simultaneously to adder 305 from which the resultant sum is immediately transferred to sum buffer 310 for short-term storage.
  • Registers 116, 121 and transceivers I05, 106 are reset during the next 20- nanosecond interval. Then in the final 40 nanoseconds, the composite signal sample is transferred from sum buffer 310 to both transceivers I and 106 via the same transmission paths occupied during the initial transfer from the transceivers to the highway registers.
  • the composite signal is applied to subtractors 300 and 301 such that the signal sample derived from station 100-1 is converted to analog form in decoder 104 and applied to station l00-n. Similarly, the signal sample derived from station l00-n is converted to analog form in decoder 102 for application to station 100-1.
  • This sequence of operations requires 20 nanoseconds more than utilized in the arrangement according to FIG. 1.
  • the sampling rate of 8 kilohertz the number of time slots which can be accommodated by switch store I50 is reduced to 893.
  • the arrangement according to FIG. 3 is particularly adaptable to multiparty conference connections.
  • four telephone lines 500-503 are interconnected for a conference.
  • additional sum buffer 510 is connected to receive the cumulative signal sum stored in buffer 310.
  • n conferees n-l time slots are required if n is even, and n time slots are required if 503 is odd.
  • an additional conference time slot is employed, as illustrated in FIG. 5.
  • the composite signal is retained in buffer 310 for transfer to lines 500 and 501 during the third conference time slot via bidirectional gates I07 and 521. All conference time slots appear in sequence during the repetitive cycle of time slots.
  • FIG. 6 An alternative arrangement for multiparty conferences is illustrated in FIG. 6.
  • an additional memory register 601 and translators 602 and 603 are included.
  • the bidirectional gate control signals in memory 150 are applied to translators 130 and BI during the first conference time slot in order to transfer signals from lines 500 and 501 and simultaneously, these gate control signals are stored in register 60].
  • the resultant composite signal may be gated simultaneously to all conference lines 500-503 by applying the contents of registers M0 and 601 simultaneously to appropriate ones of the bidirectional gates.
  • Combining circuits 600 and 604 assure that the composite signal will be transmitted to each of the conference lines as required.
  • This alternative requires additional circuitry but has the advantage of conserving available time slots where speed is of the essence.
  • Underlining the conference arrangements considered in this illustrative embodiment of our invention is the transfer of signal samples in digital form rather than in the analog form as employed in the prior art.
  • the virtue of digital transfer is the preservation of the same signal level throughout the transfer operation for all conferees.
  • the energy in the analog signal is divided among all conferees.
  • the energy of his voice is divided equally between the two recipients, assuming, of course, that the line circuits of all conferees are perfectly terminated.
  • Each additional conferee will reduce the signal level accordingly. In such an arrangement it is impractical to permit more than four conferees in a single conference without inserting additional gain.
  • any number of conferees may be included in the conference connection without signal loss or degradation.
  • each of the two highways I15 is provided with its own adder and buffer circuitry, viz, adder 700, sum buffer 701i and additional sum buffer 702 for highway 115 and adder 710, sum buffer 711 and additional sum buffer 712 for highway 120.
  • the switch store is divided into two equal sections 150 and E50 with separate access to each section, thus forming two independent stores of the same word length. The two stores are run in synchronism with translator 130 associated with section 150 and translator 131 associated with section 150'.
  • the other highway and the associated section of the switch store is available to continue the operation of the system on its own. For example, if highway 115, adder 305 and switch store section 150 all exhibit failures simultaneously, the system may be continued in service utilizing highway 120, adder 710 and switch store section 150'.
  • FIGS. 8 and 9 Alternative arrangements for maintaining this in-service" condition are shown in FIGS. 8 and 9.
  • three time slots are assigned to each conversation which, of course, means that the system capacity is reduced to one-third of its normal capacity.
  • a sample from subscriber line 500 is stored in additional sum buffer 712 during the first time slot, and in the second time slot a sample from line 503 is added to the sample in buffer 712 and the resultant sum returned to line 503.
  • the resultant sum contained in bufier 711 is returned to line 500.
  • the alternative method provides a memory buffer 901 and additional translator 902 which permits continued service on a two-time slot per conversation basis. Again a sample from line 500 is stored in additional sum buffer 712 during the first time slot, and at the same time the information for enabling bidirectional gate 109 is transferred to memory buffer 901. In the second time slot, therefore, the signal from line 503 is added to the contents of buffer 712 and the resultant sum applied to both lines 500 and 503 simultaneously via gates 107 and 108.
  • a time division switching system comprising a plurality of lines, first and second common transmission highways and means for interconnecting calling and called ones of said lines via said highways during an assigned time slot in a repetitive cycle of time slots, said interconnecting means comprising means for simultaneously transferring a signal sample from each of said calling and called lines to said first and second highways respectively, means for adding said calling line sample to said called line sample, means for transferring the resultant signal summation to said calling and called lines, means for subtracting the signal sample developed in each of said calling and called lines from said resultant signal summation and means for applying the resultant difference signals to the respective calling and called lines.
  • a time division communication system comprising a plurality of lines, a plurality of common highways accessible from said plurality of lines, means for transferring a signal sample from each of said plurality of lines to a corresponding preselected one of said plurality of highways during a first portion of a time slot in a repetitive cycle of time slots and means operative in a second portion of the same time slot for trans ferring said signal samples from said plurality of highways to corresponding, predetermined ones of said plurality of lines.
  • each of said highways comprises register means, said signal samples from said plurality of lines being interchanged via said register means during said same time slot.
  • each of said plurality of lines comprises means for coding and decoding said signal samples and further comprising means for registering said coded signal samples prior to transmission to said highways via said transferring means.
  • each of said highways comprises register means, and further comprisin means interconnecting said highway register means for com inmg said signal samples and buffer storage means for applying the resultant combined signal sample from said combining means to each of said plurality of lines via said register means in each of said highways, and wherein each of said lines comprises means for substracting the signal sample produced by the corresponding line from said resultant combined signal sample.
  • a time division communication system in accordance with claim 5 and further comprising means for storing the content of said bufier storage means and means for applying the content of said storing means to said combining means during a succeeding time slot.
  • a communication system comprising a plurality of stations, common highway means, means for coding signal samples from the associated stations, means for storing said signal samples, switching means, and means for enabling said switching means during distinct successive time intervals in a repetitive cycle to transfer said signal samples to and from said highway means, said coding means being connected between said storing means and each of said stations, said switching means being connected between said storing means and said highway means, and said highway means comprising register means for storing said coded signal samples intermediate transfers to and from said highway means.
  • said highway means comprises a pair of common highways, and further comprising means interconnecting said highway register means for adding together said stored samples and means for applying the resultant sum to said highways.
  • this resultant composite signal is transferred from buffer 310 to conference lines 502 and 503 via the same bilateral gates 5'29 and 108 from which the signal samples previously were--.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Use Of Switch Circuits For Exchanges And Methods Of Control Of Multiplex Exchanges (AREA)
  • Time-Division Multiplex Systems (AREA)
US844945A 1969-07-25 1969-07-25 Time division switching system employing common transmission highways Expired - Lifetime US3617643A (en)

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US (1) US3617643A (fr)
BE (1) BE753908A (fr)
DE (1) DE2036796C3 (fr)
FR (1) FR2055593A5 (fr)
GB (1) GB1322978A (fr)
NL (1) NL7010708A (fr)
SE (1) SE372687B (fr)

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3727006A (en) * 1970-02-10 1973-04-10 Cit Alcatel Multi-stage time connection network
US3737586A (en) * 1971-10-01 1973-06-05 Bell Telephone Labor Inc Time division switching system
US3761624A (en) * 1972-07-31 1973-09-25 Bell Telephone Labor Inc Time division signal transfer network
US3787630A (en) * 1972-07-31 1974-01-22 Bell Telephone Labor Inc Time division communication system
US3804989A (en) * 1972-07-31 1974-04-16 Bell Telephone Labor Inc Time division communication system
US3825693A (en) * 1972-09-25 1974-07-23 Tele Resources Inc Time division multiplex branch exchange
US3825690A (en) * 1972-06-01 1974-07-23 Gte Automatic Electric Lab Inc Lossless network junctor for pcm digital switching systems
DE2405401A1 (de) * 1973-03-01 1974-09-19 Ibm Zeitmultiplex-vermittlungseinrichtung
US3903372A (en) * 1973-06-05 1975-09-02 North Electric Co Time division multiplex conferencing system
US3940564A (en) * 1973-10-29 1976-02-24 Tadiran Israel Electronics Industries Ltd. Time division multiplex system with conference link
US3959594A (en) * 1974-07-01 1976-05-25 Gte Automatic Electric Laboratories Incorporated Arrangement and method for the localized self-control of randomly allotted time slots to audio ports
DE2535506A1 (de) * 1975-08-08 1977-02-17 Magnavox Co Verfahren und einrichtung zum zusammensetzen von in digitaler form dargestellter information
US4456991A (en) * 1981-09-18 1984-06-26 International Telephone And Telegraph Corporation Telephone line circuit and system

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2398416A2 (fr) * 1975-07-30 1979-02-16 Siemens Ag Procede et montage pour la conversion de signaux analogiques en signaux numeriques et de signaux numeriques en signaux analogiques

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3225144A (en) * 1962-05-16 1965-12-21 Bell Telephone Labor Inc Telephone switching system

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3225144A (en) * 1962-05-16 1965-12-21 Bell Telephone Labor Inc Telephone switching system

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3727006A (en) * 1970-02-10 1973-04-10 Cit Alcatel Multi-stage time connection network
US3737586A (en) * 1971-10-01 1973-06-05 Bell Telephone Labor Inc Time division switching system
US3825690A (en) * 1972-06-01 1974-07-23 Gte Automatic Electric Lab Inc Lossless network junctor for pcm digital switching systems
US3787630A (en) * 1972-07-31 1974-01-22 Bell Telephone Labor Inc Time division communication system
US3804989A (en) * 1972-07-31 1974-04-16 Bell Telephone Labor Inc Time division communication system
US3761624A (en) * 1972-07-31 1973-09-25 Bell Telephone Labor Inc Time division signal transfer network
US3825693A (en) * 1972-09-25 1974-07-23 Tele Resources Inc Time division multiplex branch exchange
DE2405401A1 (de) * 1973-03-01 1974-09-19 Ibm Zeitmultiplex-vermittlungseinrichtung
US3903372A (en) * 1973-06-05 1975-09-02 North Electric Co Time division multiplex conferencing system
US3940564A (en) * 1973-10-29 1976-02-24 Tadiran Israel Electronics Industries Ltd. Time division multiplex system with conference link
US3959594A (en) * 1974-07-01 1976-05-25 Gte Automatic Electric Laboratories Incorporated Arrangement and method for the localized self-control of randomly allotted time slots to audio ports
DE2535506A1 (de) * 1975-08-08 1977-02-17 Magnavox Co Verfahren und einrichtung zum zusammensetzen von in digitaler form dargestellter information
US4456991A (en) * 1981-09-18 1984-06-26 International Telephone And Telegraph Corporation Telephone line circuit and system

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FR2055593A5 (fr) 1971-05-07
DE2036796B2 (de) 1980-06-04
SE372687B (fr) 1974-12-23
GB1322978A (en) 1973-07-11
BE753908A (fr) 1970-12-31
DE2036796A1 (de) 1971-02-04
NL7010708A (fr) 1971-01-27
DE2036796C3 (de) 1981-02-12

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